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Posted: 10:31 AM- The sod house pioneers who fashioned farms from the tall-grass prairies of the American Midwest probably would have scoffed had you called them visionaries.

Yet many of the natural components they used for their dwellings, including wildflowers and grasses and thick blocks of turf-covered soil that insulated everything indoors, are the stuff of today's popular green-roof movement.

As in those earlier times, green-roof construction now is driven as much by its environmental benefits as by adding eye appeal to otherwise drab properties.

"It's an ecological response to urban areas," said Edmund Snodgrass, co-author with his wife, Lucie, of Green Roof Plants: A Resource and Planting Guide. These roof gardens help manage storm rainfall and insulate against city heat buildup, he explained.

In pioneer days, "They didn't have a heating and air conditioning guy in the neighborhood back then," Snodgrass said. "They had to find a way of using nature to their advantage. Now we're doing the same thing, but we're moving intentionally away from total dependence on mechanical systems."

Builders classify modern green-roof designs as "intensive" or "extensive."

Intensive roof gardens require a great deal of structural support for such things as trees and trails, seating and a growing medium more than 6 inches deep. They also require irrigation and frequent maintenance, in showcasing trees, shrubs and flowers.

An extensive garden, meanwhile, is built on a thin layer of growing material, requires little to no maintenance and supports drought-tolerant plants capable of surviving long periods of heat or winter cold.

Snodgrass owns and operates Emory Knoll Farms Inc. near Street, Md., which he described as the first nursery in the United States dedicated solely to the propagation and sale of green roof plants.

Roof gardens are built primarily over the flat roofs of large commercial buildings and plant sites, but growing numbers of residential developers are beginning to take an interest, Snodgrass said.

The green-roof phenomenon began to spread in the 1950s when some German cities encouraged building owners to convert their tar and pebble rooftops to vegetation. Since then, Germany has developed more than 50 square miles of green roof space and adds an additional five square miles per year, according to Christian Werthmann, an assistant professor of landscape architecture at Harvard's Graduate School of Design.

The U.S. has only a fraction of the green roof space found in Germany but a recent study by Green Roofs for Healthy Cities determined that green roof space in this nation grew 80 percent in the past couple of years.

Chicago leads the trend among all U.S. cities, having planted 300,000 square feet of green roof space in just one year, the non-profit industry group said. That includes putting a vegetative cover over city hall.

The new Ford Rouge Dearborn, Mich., truck assembly plant has what its architects say is the largest "living roof" in the world, a 10-acre surface planted with a mixture of nine low-maintenance sedum varieties.

Now, Snodgrass said, green roofs are being seen increasingly on urban residential buildings like condominiums, which are more flat-roof. "The single-family home with a pitched roof is still a little ways off.

"People are tending to use green roofs more on additions and garages rather than on whole roofs. You can cut a door into the roof to give it some additional utility," he said. "A roof turned garden gives your family some extra living space."

Upfront costs may be higher ($10 to $20 a square foot vs. $5 to $10), but green roofs conservatively can be expected to last two to three times longer than a conventional roof, according to the American Society of Landscape Architects. They also can reduce the heating and cooling costs for buildings by at least 10 to 15 percent, says the society, which installed a demonstration garden on its Washington headquarters roof a year ago.

Green roofs also can retain up to 75 percent of a 1-inch rainfall, greatly reducing the pressure storm water puts on municipal sewer systems, the society says.

Plant selection for a roof garden varies according to geography, soil depth and type, light conditions, accessibility and personal tastes.

"There's a difference in conditions between Nova Scotia and Phoenix, of course, and you have to learn to appreciate that," Snodgrass said. "Be aware of anything you put on the roof that could dry down (go dormant) and be ignited."

Some green-roof layouts are more functional than others. The roof of a hotel in Vancouver, British Columbia, for example, is planted with an assortment of kitchen herbs used by its restaurants.

"I know of one in a Boston hospital that's built just outside of one of the cancer wards," Snodgrass said. " ...It makes for a great healing garden."

Aside from providing color and beauty in generally unused settings, green roofs also can furnish cover for a variety of wildlife species.

"Green roofs won't bring back our elk and bison herds but they will attract larvae and butterflies and other pollinators, including honeybees," Snodgrass said. "Killdeer and many different birds also nest on these roofs."

For more about green roofs, see this University of Florida Extension Service Web site:

It may seem like a no-brainer but, just for the record, access is everything if you're planning to install a green roof.

If you envision a low-care, no-care kind of cover crop, fine. Plant your rooftop garden with an assortment of succulents and forget about it. Most of those drought-tolerant, winter-hardy plants can look after themselves.

But if you want the meadow look for your roof, then expect to do some weeding, watering, deadheading and replanting. For that, you will need a way to get there, and that's not a design feature built into many office buildings or homes.

Of the $950,000 spent to build a demonstration green garden on the roof of the American Society of Landscape Architects' headquarters building in Washington, $600,000 went for a new interior stairway and landing to provide visitor access, officials said.

"There's a great deal of difference between a highly viewed garden and a purely functional garden," said Edmund Snodgrass, co-author of Green Roof Plants: A Resource and Planting Guide (Timber Press, 2006).

"Succulents are the most popular," said Snodgrass, who also owns and operates Emory Knoll Farms and Green Roof Plants near Street, Md. The business supplies plants for over a million square feet of green roofs in 20 states and the District of Columbia.

"The best quality of the (succulent) plant is that it lives on existing rainfall. If you get into a lot of irrigation or maintenance, it just drives the cost of the roof up," he said. "Look for plants that can withstand drought, wind and all the things that make surviving on roofs more harsh. That points toward succulents."

Shop around for plants that are both attractive and practical. Many of the sedums are shallow-rooted and well suited to roof plantings. Certain cactus varieties are rugged enough to make it in a thin, inorganic medium. And they don't go dormant, either, meaning they provide color all year.

If your plant material is 4 inches deep or more, then consider adding some herbaceous perennials (Phlox, Dianthus, Campanula, Salvia and Potentillas, among others). Kitchen herbs can be a smart addition if you have a half-foot or more of soil depth. Grasses are good, too, although some may need to be mowed and irrigated, and they can become a fire hazard when allowed to dry.

Snodgrass' basic advice:

- Use plugs, two per square foot. They'll grow into a vegetative roof in about a year and a half.

- Succulents, or "little canteens," as he calls them, grow readily from cuttings. "It takes a while, but it's inexpensive."

- The more juvenile the plant, the better. "They become more adapted to the roof environment."